We report the structural and magnetic properties of rare-earth substituted spin-chain compounds Ca${}_{2.75}$$R$${}_{0.25}$Co${}_2$O${}_6$ ($R$ $=$ Dy and Lu). The Rietveld refinement of neutron and x-ray powder diffraction patterns confirms the single-phase formation of both compounds in the rhombohedral structure (space group $R$$\overline{3}$$c$). The derived values (from the analysis of the neutron diffraction patterns at 50 K) of the bond-valence sum indicate a reduction in the oxidation state of the cobalt ions at the trigonal prism (TP) site (6$a$) with $R$ substitution, which is further supported by low temperature neutron diffraction [where a zero value of the ordered moment at the 6$b$ site and a reduction in the values of the maximum ordered moment at the TP site have been observed] and dc magnetization studies. In the neutron diffraction patterns, additional Bragg peaks appear for both compounds below Néel temperature ($T_N$) of $\sim$16 K, indicating the onset of an antiferromagnetic ordering of cobalt spin chains on the triangular lattice. The magnetic structure corresponds to a spin density wave (SDW) structure [with a propagation vector $\mathit{k}=\{0,0,1.02\}$], having $c$ axis as a direction of both moment and modulation. For both compounds, the refined values of the ordered moment at the 18$e$, 6$b$, and 6$a$ sites are $\sim 0.03\left(2\right),0.02\left(2\right)$, and 4.2 (2) $\mu$${}_B$, respectively. Unlike the parent compound Ca${}_3$Co${}_2$O${}_6$, no temperature dependence as well as no time dependence in the intensity of the strongest antiferromagnetic reflection (10 $\tau$), corresponding to the propagation vector $k$ $=$ {0, 0, 1.02}, has been observed down to 1.5 K confirming that the SDW structure is stabilized by the substitution with rare-earth ions. The stabilization of the SDW structure and the observed decrease in the values of $T$${}_N$ could be due to a decrease in the value of positive FM intrachain exchange interaction $J$ with the rare-earth substitution in a system with competing intrachain and interchain exchange interactions.

• Received 27 December 2012

DOI:https://doi.org/10.1103/PhysRevB.87.094411